HEAT TREATMENT. 299 '
The foregoing argument may be summarized as stated by Sau-Veur: •
(1) All unhardened steels are composed of pearlite alone, or of' pearlite associated with ferrite or cementite. :
' (2) Without taking into consideration austenite and troostite,' hardened steel is composed of martensite alone, or of martensite associated with ferrite or cementite.
(3) Ferrite and cementite cannot exist together in the same piece of steel.
(4) The presence of the lamellar variety of ,pearlite is almost certain proof that the steel has been annealed.
Following the proposition that ferrite is iron free from carbon and that cementite is a, compound represented by the formula, FesC, it is evident that in very low steels, say ranging from .02-.10 carbon, the structure will be almost'entirely ferrite, and that in steel of 2.00 per cent, carbon there will be an excess of cementite. There will therefore be one point of carbon content at which the component ferrite and cementite will both be satisfied, which is to say that the.original proportion will be/that of the eutectic alloy. This occurs in a pure steel containing about .80 per cent, of carbon, the micro-structure of this grade showing no ferrite or cementite.
Late .investigations seem to prove that in hypereutectic steels,, that is, those containing more than' .89 per cent, of carbon, the upper critical point, A3, follows the curve, SE, in Fig. XV-H. This is the point at which cementite begins to form and, according to Howe and Eoberts-Austen,... progressively separates out within the martensite in cooling, and forms a network whose coarseness is proportional to the temperature to which the steel has been heated. No break in the cooling curve has been noticed, but the first appearance of cementite is considered to mark the point, Ar3, while Ar2 and Ar-t are as given in diagram Fig. XV-A.
.Tables taken from Prof. Sauveur give results as shown in Tables XV-L and XV-M, the numerals being intended to represent per cent, of volume, since if a body containing an infinite number of particles, uniformly distributed, is cut by a plane; the ratio of the? sum of the small-areas to the total area is equal to the :ratib of the* volume -of the'small particles to the total volume. Theoretically,"• of course, this is not true of a mass of steel, but for practical poses 'it islicorrec'f.; ;v ". ' ' ' ' v; •• ; :'- •''. ' '••:" "::"